Information technology (IT) systems typically fill a vital role in the functioning of an enterprise. Failure of a system is frequently unacceptable, and many operations benefit from redundancy, so that a redundant component can quickly take the place of a failed component. Constructing duplicates of a component leads to considerable expense, and one approach to information services and processing that can increase the flexibility of a system and decrease the expense of providing redundancy is the use of virtual machines. A virtual machine does not require that a single specific hardware element be configured to carry out the virtual machine's operation. Instead, one or more hardware elements are configured to carry out the operations of a virtual machine, with these operations being carried out by a single hardware element or spread across a plurality of hardware elements. A virtual machine can be moved from one hardware element or combination of hardware elements to one another, and a hardware element that is running a virtual machine can be redirected to other purposes. In addition, if the data used to construct a virtual machine is available, a new virtual machine can be constructed in a relatively short time, frequently using automated mechanisms, so that a virtual machine can be replaced without intervention and without expense for hardware replacement.
One mechanism for providing redundancy for information services is through the use of a high availability system, in which a hardware element, such as a server, belongs to a high availability cluster. A high availability cluster comprises a plurality of nodes that have access to data identifying the network elements being served by one or more of the nodes. The high availability cluster implements a failover strategy that allows for transfer of operations from a failed component.
One mechanism for providing a high availability system is to use virtual machines for the components for which redundancy is to be provided. A virtual machine can be restored by copying the data comprising the virtual machine to a suitable hardware element or combination of hardware elements. A virtual machine may comprise a plurality of logical unit numbers (LUNs) that comprise components of the virtual machine. A LUN is a number used to identify a logical unit which is a device addressed by the small computer system interface (SCSI) protocol or similar protocols such as Fiber Channel or internet small computer system interface (iSCSI) A LUN may be used with any device which supports read/write operations, such as a tape drive, but is most often used to refer to a logical disc.